There's a popular theory that bird and mammal evolution kicked into high gear after the dinosaurs went extinct. But now it turns out lice were already diversifying long before the dinosaurs died out.

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This is significant because lice would only start to diversify when their various hosts - in other words, birds and mammals - were themselves starting to diversify into lots of different species. We know that, in the last hundred million years, birds and mammals have spread into almost every available ecological niche, and most researchers assumed that this happened shortly after the dinosaurs went extinct 65 million years ago, thus opening up all this prime ecological real estate.

But now, new research by University of Illinois ornithologist Kevin Johnson suggests that lice started diversifying before the extinction event at the end of the Cretaceous. This, as he explains, means big things for bird and mammal evolution:

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This study lends support to the idea that major groups of birds and mammals were around before the dinosaurs went extinct. If the lice were around, we know their hosts were probably around. Ducks do different things from owls, which do different things from parrots, for example, and it was thought that after the dinosaurs went extinct that's when these birds or mammals diversified into these different niches. But based on the evidence from lice, the radiation of birds and mammals was already under way before the dinosaurs went extinct.

Lice really do undergo some dramatic evolutionary transformations to better fit with their preferred host. Gopher lice, for instance, have special grooves on the tops of their heads that allow them to hold onto a single hair, while wing lice have unusually long bodies that allow them to squeeze into the barbs of the bird's feather. These adaptations essentially lock the lice into one particular host species, meaning they would only happen in the presence of mass diversification.

Beyond the basic elegance of the idea that birds and mammals started diversifying after the dinosaur extinction, there was also the fact that the oldest fossils that resemble modern species all date back to less than 65 million years ago. The newly found lice fossils are only indirect evidence of earlier diversification, to be sure, but it's hard to reconcile these lice fossils with the current paradigm. If radiation did begin before the dinosaurs died out, it also means we have to come up with some new explanation for why birds and mammals started diversifying in such great numbers.

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Johnson also tackles another crucial question...did dinosaurs themselves have lice?

"Our analysis suggests that both bird and mammal lice began to diversify before the mass extinction of dinosaurs. And given how widespread lice are on birds, in particular, and also to some extent on mammals, they probably existed on a wide variety of hosts in the past, possibly including dinosaurs. So maybe birds just inherited their lice from dinosaurs."